Computing systems generally use one or more display monitors to provide a visual input/output capability. While the structure of computer display monitors varies, all generally include a cathode ray tube (CRT) having an evacuated envelope usually made of high-strength glass. The envelope defines a generally flat or slightly curved faceplate together with a funnel shaped bell and extending neck. The interior side of the faceplate supports a phosphor display screen. In monochrome displays, a single electron gun is supported within the CRT neck and is directed toward the phosphor screen. The electron gun produces a beam of electrons which are directed toward the faceplate striking the phosphor screen and causing visible light to be emitted therefrom. In color display systems, a plurality of electron guns are used together with a phosphor screen which supports plural areas of phosphors having differing color light emitting characteristics. A shadow mask or similar structure is interposed between the electron guns and the phosphor screen to cause each of the electron guns to stimulate an associated type of colored light emitting phosphor.
whether the display system is monochrome or color, the electrons emanating from the electron gun or guns form a CRT beam which is scanned in both the horizontal and vertical directions across the faceplate to form a raster. In most instances, the horizontal scan system is operative at a higher frequency than the vertical scan system. Thus, the horizontal scan moves the electron beam rapidly from side to side across the faceplate while the vertical scan system causes the successive horizontal scans to be moved progressively from top to bottom to complete a display frame and form the raster.
In the majority of the presently used display systems, electron beam scanning is accomplished by electromagnetic deflection of the CRT beam. A deflection yoke is supported upon the CRT envelope between the electron guns and the faceplate. The deflection yoke supports a plurality of deflection coils which are coupled to the horizontal and vertical scan systems. Horizontal and vertical scan signals provided by the respective scan systems are coupled to the windings of the deflection yoke to produce corresponding electromagnetic fields which bend the electron beam and thereby direct it to the desired portion of the CRT faceplate. Both the horizontal and vertical scan signals include longer duration sloped scan portions followed by shorter duration high amplitude retrace portions. The latter are utilized at the completion of each respective scan interval to return the electron beam to its starting position. In addition, the retrace portion of the horizontal scan signal is used to develop the high voltage necessary to accelerate the electron beam toward the CRT faceplate.
The character of the image displayed in a CRT display system results from variation or modulation of the intensity of the scanned CRT electron beam. This intensity modulation must be properly timed or synchronized to the horizontal and vertical rate scanning of the raster. Thus, as the electron beam is scanned across the faceplate to form a raster, the desired portions of the faceplate are illuminated by synchronized modulation of the electron beam to provide the desired image.
In most display monitors, a single image size format is presented and great care is taken to establish and maintain a precise relatively constant image size. However, it has been found desirable in certain applications to include the capability for expansion or enlargement of the displayed image to produce an enhanced more dramatic effect. In addition, such expansion or enlargement of the displayed image also renders the material easier to read and view. Image expansion is typically accomplished by deliberately inducing an "overscan" condition within the display monitor. Most monitors achieve overscan by simultaneous increase of the vertical deflection currents driving the deflection yoke. In effect, the raster is scanned beyond the border or outer edge of the CRT faceplate. Systems having overscan capability are subject, however, to additional problems not encountered by single display size monitors. For example, the precise adjustment and maintenance of the multiple size formats is often difficult to achieve. Frequently, the size changing mechanism introduces inaccuracies in the size control systems tending to degrade overall performance of the display monitor. In particular, the horizontal deflection system of display monitors often renders multiple size format operation more difficult due to the plurality of highly interactive tuned circuits within the deflection system and the interactive relationship between horizontal scan and high voltage generation.
There remains, therefore, a need in the art for an inexpensive, efficient image expanding display system operable upon the horizontal deflection systems of monitors which effectively provides for independent image size adjustment in each operating mode.
Accordingly, it is a general object of the present invention to provide an improved display monitor. It is a more particular object of the present invention to provide an improved display monitor having image expanding capability which substantially maintains image size stability in each mode of operation. It is a still more particular object of the present invention to provide a horizontal scan system for use in a display monitor which is operable in an image expanded mode and which is efficient and relatively inexpensive.